In a nuclear reactor of boiling water type, a plurality of control rods is located at a reactor core. Furthermore, at a lower part of a pressure vessel of the nuclear reactor, a control rod-driving mechanism for driving each control rod with insertion/withdrawal by high-pressure driving water is installed respectively.
In this control rod-driving mechanism, the high-pressure driving water is supplied from a driving water-supply device. Then, the driving water exhausted from the control rod-driving mechanism is returned to the driving water-supply device again. Component of this control-rod driving mechanism will be explained by referring to FIGS. 3 and 4. Here, among a plurality of control rods, one control rod and a driving mechanism corresponding thereto will be explained.
As shown in FIG. 3, a control rod 1 is connected to a piston 3 of a control rod-driving mechanism 20. In case of settling the control rod 1, the control rod 1 is held by a latch 4 set to a control rod-guide tube 2. Only when a load is applied along a direction to withdraw the control rod 1, this latch 4 is moved to hold the control rod 1 while the latch 4 does not disturb insertion motion of the control rod 1. In order for the control rod 1 to settle at a plurality of positions, a plurality of latches 4 is located along a longitudinal direction of the guide tube 2.
As to the driving water to give a pressure to an upper part or a lower part of the piston 3, a pressure thereof is risen by a driving water pump 5. Then, the driving water is supplied to the upper part of the piston 3 via a control valve for withdrawal direction (driving water side) 6, or the lower part of the piston 3 via a control valve for insertion direction (driving water side) 7. In this case, the driving water at the pressure side by motion of the piston 3 is exhausted via a control valve for withdrawal direction (exhaust water side) 8 or via a control valve for insertion direction (exhaust water side) 9.
When the control rod 1 is driven to be inserted, by opening the control valve for insertion direction (driving water side) 7 and by closing the control valve for withdrawal direction (driving water side) 6, the driving water is supplied to the lower part of the piston 3. At the same time, by opening the control valve for insertion direction (exhaust water side) 9 and by closing the control valve for withdrawal direction (exhaust water side) 8, the driving water of the upper part of the piston 3 is exhausted. As a result, the piston 3 is moved to an insertion direction, and the control rod 1 is driven to the insertion direction.
After driving the control rod 1 to be inserted at a predetermined distance, when the control rod 1 is to be settled, the control valve for insertion direction (driving water side) 7 and the control valve for insertion direction (exhaust water side) 9 are closed, and the control valve for withdrawal direction (exhaust water side) 8 is opened. As a result, the piston 3 is moved to the withdrawal direction. Here, by applying a load to the latch 4, the latch is held (settled) at a predetermined position.
When the control rod 1 is driven to be withdrawn, by opening the control valve for insertion direction (driving water side) 7 and the control valve for insertion direction (exhaust water side) 9, holding of the control rod 1 by the latch 4 is released (unlatch). Next, after closing the control valve for insertion direction (driving water side) 7 and the control valve for insertion direction (exhaust water side) 9, by opening the control valve for withdrawal direction (driving water side) 6, the driving water is supplied to the upper part of the piston 3. At the same time, by opening the control valve for withdrawal direction (exhaust water side) 8, the driving water of the lower part of the piston 3 is exhausted. As a result, the piston 3 is moved to a withdrawal direction, and the control rod 1 is driven to the withdrawal direction.
After driving the control rod 1 to be withdrawn at a predetermined distance, while the control valve for withdrawal direction (exhaust water side) 8 is being opened, the control valve for withdrawal direction (driving water side) 6 is closed. As a result, the piston 3 is moved to the withdrawal direction. Here, by applying a load to the latch 4, the latch 4 is held (settled) at a predetermined position.
In response to a control rod-selection command or a withdrawal/insertion-driving command from an operator, a control rod-driving control apparatus 10 outputs an open and close command to each control valve 6˜9 of the control rod-driving mechanism 20 corresponding to the selected control rod 1. Here, a timing to open and close each control valve 6˜9 when the selected control rod 1 is operated (such as unlatch, insertion, withdrawal, settle) is previously set according to a distance to insert or withdraw the selected control rod 1. FIG. 4A is an example of a timing table when the control rod 1 is driven to be inserted, and FIG. 4B is an example of a timing table when the control rod 1 is driven to be withdrawn. By operating open and close of each control valve according to timing tables shown in FIGS. 4A and 4B by the operator, the selected control rod is driven to be inserted or withdrawn at a predetermined distance.
For example, in the example of the timing table (when the control rod is driven to be inserted) shown in FIG. 4A, when 0.4 seconds have passed from a reference time, the operator opens the control valve for insertion direction (driving water side) 7 and the control valve for insertion direction (exhaust water side) 9. When 3.3 seconds have passed from the reference time, the operator closes both control valves, and opens the control valve for withdrawal direction (exhaust water side) 8. When 8.6 seconds have passed from the reference time, the operator closes the control valve for withdrawal direction (exhaust water side) 8. While these control valves 7, 8, 9 are being operated, the control valve for withdrawal direction (driving water side) 6 is continuously closed. Furthermore, in the example of the timing table (when the control rod is driven to be withdrawn) shown in FIG. 4B, the operator opens and closes each control valve 6˜9 according to driving timings of FIG. 4B.
By the way, a driving velocity of the control rod 1 is determined by a differential pressure of the driving water supplied to the guide tube 2. Here, in accordance with change of the differential pressure, the driving velocity is also changed. Accordingly, by setting a flow control valve to a pipe to supply/exhaust the driving water and by controlling the flow control valve, the differential pressure is maintained as a predetermined value. As a result, change of the driving time is prohibited. This technique is disclosed in JPA (Kokai) PH6-174878.
In above-mentioned control rod-driving mechanism of conventional technique, the velocity to insert/withdraw the control rod 1 is basically determined by the differential pressure that the driving water (supplied to the guide tube 2) operates on the piston 3. Actually, due to a difference of mechanical characteristic of the control rod-driving mechanism 20 and each control valve 6˜9 or a chronological change thereof, distribution to some extent exists among each velocity.
In order to correct the individual difference, the flow control valve is set to each control valve 6˜9 respectively. By adjusting the flow and the differential pressure, the driving velocity is controlled to be constant. However, during driving, a validation test cannot be performed. Furthermore, if the flow control valve is adjusted during activating/driving, a nuclear-thermal hydraulics characteristic of the nuclear reactor is changed. As a result, while the nuclear reactor is being activated or driven, the flow control valve cannot be adjusted, which is a problem.
Furthermore, in the conventional technique, a timing to drive each control valve 6˜9 is commonly set to all control rods 1, and cannot be set to each control rod 1 respectively. Briefly, a timing to operate unlatch, insertion, withdrawal and settle, cannot be adjusted and set to each control rod 1. Accordingly, if at least two control rods 1 are simultaneously driven, due to the difference of mechanical characteristic, dispersion exists among the driving velocity of each control rod 1. As a result, a difference occurs among moving difference of each control rod 1, and the difference is often over a permissible range. In this case, by an indication from a monitoring unit of abnormality variability, the driving operation of the control rod 1 is settled. As a result, it takes a long time for the restoration, and this situation badly affects on activation/operation of the nuclear reactor.
Furthermore, as to unlatch operation to be performed before withdrawal operation during driving the control rod, due to factors (individual difference, deterioration) of the machine side, the latch cannot be often released at the predetermined time. In this case, the unlatch time cannot be set respectively. In order to withdraw such control rod 1, the withdrawal operation must be performed a plurality of times. Briefly, complicated operation is necessary, which is a problem.